In general, ceramic materials are large in the strength at high temperature, and excellent in the heat resistance, oxidation resistance and corrosion resistance, so that they are particularly desirable as a structural material. Especially, a great interest is taken in the application of such ceramics as a structural material used in a temperature region exceeding a use limit of a metal. Among these ceramics, silicon carbide (abbreviated as SiC hereinafter) or silicon nitride (abbreviated as Si.sub.3 N.sub.4 hereinafter) is excellent in the heat resistance and oxidation resistance, so that it is very desirable as a structural material capable of using at a high temperature.
However, SiC or Si.sub.3 N.sub.4 is hardly sintered, and is a material that it is very difficult to provide a dense sintered body without adding an assistant. For this end, the firing has hitherto been conducted by adding Al.sub.2 O.sub.3 as a sintering aid to SiC powder, or by adding Y.sub.2 O.sub.3, CeO.sub.2, Al.sub.2 O.sub.3, AlN, MgO or the like as a sintering aid to Si.sub.3 N.sub.4 powder.
Therefore, the SiC sintered body obtained by using Al.sub.2 O.sub.3 or the like as a sintering aid becomes dense through liquid-phase sintering, but pores are apt to be generated owing to the reaction between Al.sub.2 O.sub.3 and SiC. As a result, the strength is not more than 600 MPa and the toughness value at break is 5 MPa.m.sup.1/2, and a m-value of Weibble distribution as an indication showing the reliability of ceramics is not more than 10, so that it could be said to be a poor reliability material.
On the other hand, the Si.sub.3 N.sub.4 sintered body obtained by using Y.sub.2 O.sub.3, CeO.sub.2, Al.sub.2 O.sub.3, AlN, MgO or the like as a sintering aid is dense and large in the strength, but is low in the toughness as compared with zirconia ceramics. As a result, the Si.sub.3 N.sub.4 sintered body having a large strength at high temperature, an excellent oxidation resistance and the like is a material having a poor reliability in use as a structural material because the toughness is still low.
On the contrary, a part of the inventors has previously proposed a technique that the strength and toughness value and reliability of the SiC sintered body are improved by composing SiC with other ceramics. For example, a technique of producing a dense SiC-rare earth oxide-alumina composite sintered body through pressureless sintering is proposed by Omori, Takei and so on in J. Am. Ceram. Soc., 65 (1982) C-92. Further, there is proposed a technique of producing a dense SiC-rare earth oxide-alumina composite sintered body by raising the firing temperature in the pressureless sintering to 2150.degree. C. to form Al metal and Si semiconductor (see J. Mater. Sci., 23 (]988) 3744-3749 by Omori and Takei).
As a means for increasing the toughness of the Si.sub.3 N.sub.4 sintered body, a technique of improving the strength and toughness value and reliability of the Si.sub.3 N.sub.4 sintered body by anisotropically growing Si.sub.3 N.sub.4 crystal is proposed by Kawashima et al (see Takeshi Kawashima, Hiromi Okamoto, Hideharu Yamamoto and Akira Kitamura, Silicon Nitride Ceramics 2, Uchidarokauho, p135-146). That is, such a proposed technique is a method of improving the toughness of Si.sub.3 N.sub.4 -rare earth oxide-alumina composite sintered body using Y.sub.2 O.sub.3 and Al.sub.2 O.sub.3 as a sintering aid by placing a green shaped body in a capsule and sintering it under a gas pressure to anisotropically grow Si.sub.3 N.sub.4 crystal.
In the SiC sintered body synthesized by the conventional technique, small defects are always existent in the inside of the sintered body, from which breakage is started, so that such a sintered body is still lacking in the reliability as a material and has a great problem in the practical use.
On the other hand, the conventional technique of producing the Si.sub.3 N.sub.4 sintered body is a method of sintering in the capsule under gas pressure, so that the material adaptable as the capsule is less. Further, the green shaped body is housed in the capsule, so that the sintering of the complicated shape can not be conducted and the production coat becomes high. Moreover, in the Si.sub.3 N.sub.4 sintered body synthesized by this method, the crystal anisotropically and largely grows, so that a large defect is existent in the inside of the sintered body, from which the breakage is started, and consequently it is still lacking in the reliability as the material and has a great problem in the practical use.
In order to solve the above problems of the conventional techniques, the inventors have mainly made studies with respect to the formation mechanism of SiC (or Si.sub.3 N.sub.4)-rare earth oxide-alumina sintered body. As a result, the following facts have been found.
That is, an oxide solid solution or an oxide compound is firstly formed from rare earth oxide and alumina at a first stage of pressureless sintering. At a second stage, SiC or Si.sub.3 N.sub.4 solutes and diffuses into the oxide to conduct the growth of SiC grains or Si.sub.3 N.sub.4 grains, which are shrunk into the dense sintered body.
In the second stage of such a formation course, SiC powder or Si.sub.3 N.sub.4 powder solutes into the oxide and at the same time causes a chemical reaction. SiC or Si.sub.3 N.sub.4 is decomposed by the reaction with alumina to generate gases such as CO, CO.sub.2, NO and the like. As a result, defects such as void, pore and the like are created in the sintered body, and the strength of the boundary between SiC or Si.sub.3 N.sub.4 and oxide lowers.
An object of the invention is to provide a high-strength and high-reliability sintered body having no defect such as void, pore and the like in the mixed oxide composite ceramics obtained by firing SiC and/or Si.sub.3 N.sub.4 and rare earth oxide--alumina oxide and to propose a novel technique on a method of advantageously producing the same.